The present invention relates to a radio, apparatus used in radio communication systems in which both talking and listening, or transmission and reception, of speech signals are simultaneously performed by using a single frequency.
The definitions of terms used in this specification will be given prior to the description of the invention. A "radio apparatus," as used in this specification, means either an apparatus with an integrated function of both a transmitter and a receiver or an apparatus with a single function of a receiver. A "transceiver" used in this specification means an apparatus with an integrated function of both a transmitter and a receiver, unless it is defined differently in some part of this specification.
Two frequencies for the two directions are generally required for duplex radio communication of transmission and reception. Needless to say, it is preferable that duplex communication is realized by using one frequency for the efficient use of frequency bands
In some fields of applications, the high quality of sounds is not necessarily important. In such cases, the following conventional method has been used for duplex radio communication with a single frequency.
A synchronizing signal generator at an originating call station generates a synchronizing signal for informing another station of switch timing between transmission and reception. A synchronizing signal detector at the incoming call station detects the synchronizing signal contained in a received signal.
The incoming call station has a transmission/reception switching unit for switching between transmission and reception in response to a timing signal generated by the incoming call station itself or the received synchronizing signal, thereby alternately performing synchronous transmission and reception. Specifically, at the originating call station, a voice signal to be transmitted is A/D converted at a predetermined time interval and stored in a memory. The stored data is read at one half the predetermined time interval, D/A converted, and supplied to a modulator during a transmission period to thus transmit the voice signal compressed in one half the actual time period. On the other hand, at the incoming call station, the received voice signal compressed in one half the actual time period is A/D converted at a predetermined time interval and stored in a memory. The stored data is read at twice the predetermined time interval, and D/A converted to obtain a voice signal expanded to the original voice signal. In this way, simultaneous transmission and reception are performed using a single frequency.
When two transceivers are operating in such a simultaneous transmission and reception with a single frequency, a third party station can monitor only voice signals transmitted from only one transceiver which is generating a synchronizing signals in addition to the voice signals, and cannot monitor voice signals transmitted from the other transceiver which operates to receive synchronizing signals. The third party station cannot grasp therefore the full communication between the two transceivers.
One example of the simultaneous transmission and reception communication system is shown in the paper titled "Simultaneous Transmitting and Receiving Method Mobile Radio Using Only One Frequency Channel", Proceedings of the 1992 IEICE (Institute of Electronics, Information & Communication Engineers) Spring Conference, Paper No. B-769, page 3-336, Mar. 15, 1992.